Magnetic fields interact with circularly-polarized light waves propagating through optical fiber under a principle known as the magneto-optic Faraday effect. Under this principle, a magnetic field will rotate the plane of polarization of circularly-polarized light waves traveling in opposite directions and thereby cause a phase shift to occur between the relative phases of the two light waves. This phase shift is known as a non-reciprocal phase shift. For example, in a fiber optic current sensing coil affected by a magnetic field, where a first light wave, having a circular polarization state, travels in one direction in the coil and a second light wave having a circular polarization state travels in the opposite direction, there will be a phase shift between the two waves called non-reciprocal phase shift. The non-reciprocal phase shift experienced by a light wave will vary depending on whether the light wave is propagating in the same direction as the magnetic field or against the magnetic field. Measurements of the non-reciprocal phase shift may then be made to determine current or magnetic fields affecting the sensing coil.
Because the non-reciprocal phase shift occurs between light waves in a circular polarization state and because the light waves are initially in a linear polarization state, a method for converting the polarization state of a light wave is needed. To convert a light wave, e.g., vector E, having an x-axis component, Ex, and a y-axis component, Ey, from a linear polarization state to circular polarization state, the wave is passed through a highly birefringent medium. A birefringent medium is a medium that has two different indices of refraction, e.g., nx and ny. Each index of refraction corresponds to a different polarization axis where the axes are orthogonal to each other. For example, nx may correspond to an x-axis and ny may correspond to a y-axis. Because of the different refraction indices, Ex will travel at a different speed than Ey. Assuming that Ex and Ey enter the birefringent medium in phase with respect to one another, the phase difference between the components, φ, at the output of the birefringent medium is as follows:   ϕ  =            2      ⁢      π      ⁢                                    (                                          n                x                            -                              n                y                                      )                    *          d                λ            ⁢      rad        =          360      ⁢                                    (                                          n                x                            -                              n                y                                      )                    *          d                λ            ⁢      degrees      where d is the length of the birefringent medium and is the wavelength of the light wave. Thus, the phase difference between an x-axis component and a y-axis component of a light wave traveling through a birefringent medium equals the difference between the indices of refraction multiplied by the length of the birefringent medium and divided by the wavelength of the light wave.
As shown in the formula, the change of polarization state is periodic through the birefringent medium. When the phase difference between Ex and Ey changes from 0 rad (0°) to /4 rad (90°), the polarization state of E changes from linear to circular when Ex and Ey are equal in magnitude.
In addition, the change of polarization state is directly proportional to the length of the birefringent medium. With all other variables being constant, the length of the birefringent medium dictates the phase difference. The relationship between one wavelength of a given frequency of light and the length of the birefringent medium is referred to as the birefringence beat length, Λ where   Λ  =            λ                        n          x                -                  n          y                      .  
Thus, the beat length equals the wavelength divided by the difference between the indices of refraction of the birefringent medium. In other words, the physical length corresponding to one beat length of a birefringent medium corresponds to 2 π of phase shift of the light passing through that medium.
One type of birefringement medium that is typically used is know as a quarter-wave plate. One of the effects of a quarter-wave plate is to change the polarization state of a light wave from a linear polarization state to a circular polarization state. The length of a quarter-wave plate is such that components of a light wave are 90° out of phase with respect to one another upon exiting the quarter-wave plate. In particular, when a light wave is in a linear state of polarization being oriented at 45° from its principal axes, i.e., having equal components on its principal axes, and is input into a quarter-wave plate, the output is the light wave in a circular state of polarization. In a quarter-wave plate, d can be determined as follows:   d  =      Λ    ⁢                  (                              2            ⁢            m                    +          1                )            4      where m is an integer, including zero. Therefore, the length d of a birefringent quarter-wave plate is one quarter or three quarters of the beat length Λ longer than an integral number of beat lengths.
Previously, other methods have been used to convert between linear polarization states and circular polarization states. One such method uses a bulk optic quarter-wave retarder. In the case of a bulk optic device or crystal, a linearly-polarized light wave travels from a first optical fiber through a lens to collimate the light wave. The light wave then travels through a bulk optic crystal having principal axes oriented orthogonally with respect to each other and oriented at 45° with respect to the principal axes of the optical fiber. The wave then travels through a second lens and into a second optical fiber. This method is relatively costly, complex and its components occupy a relatively large amount of space. In addition, bulk optic devices are not reliable over time and temperature.
An alternate method includes the use of a single mode non-polarization maintaining fiber loop. The size and orientation of the loop converts a linear polarization state of a light wave into a circular polarization state. However, the single mode fiber loop may be hard to manipulate in achieving a desired orientation and its performance tends to degrade with temperature changes.
Thus, there is a need for a method which converts the polarization state of a light wave which eliminates or substantially reduces the disadvantages associated with prior methods.